The present invention relates to a key button structure for use in a handheld mobile phone and its similar instrument, and more specifically to a key button switch structure which can be incorporated in a handheld mobile phone and its similar instrument, and which comprises an input button having a projection, two electrodes formed on a substrate, and a dome-like switch contact to be pushed down by the projection of the input button when the input button is depressed, thereby to electrically contact both the two electrodes, and a method for forming such a key button structure.
Referring to FIG. 8, there is shown a diagrammatic section view of one example of this type key button switch structure realized in the prior art. The shown example includes an input button 21 having a projection 22, two electrodes A 29 and B 30 formed on a substrate 28, and a dome-like switch contact 27, which are located as shown. The input button 21 is formed integrally with a button connection part 25 so that when the input button 21 is depressed, the projection 22 pushes down the dome-like switch contact 27 to cause the dome-like switch contact 27 to electrically contact both the two electrodes A 29 and B 30.
FIG. 8 illustrates a typical example of the above mentioned prior art key button switch structure in which a center line 31 of the input button 21 is coincident with a center line 32 of the dome-like switch contact 27. Accordingly, when a depressing pressure or force is applied to the input button 27, the dome-like switch contact 27 is collapsed in an ideal fashion as shown in FIG. 9, with the result that an electrical interconnection is obtained between the two electrodes A 29 and B 30. In FIG. 9, the arrows “a” and “b” diagrammatically illustrate the input pressure.
On the other hand, referring to FIG. 10, there is a shown a diagrammatic sectional view illustrating an imaginary prior art example imaged by the inventors, in which the center line 31 of the input button 21 is not coincident with the center line 32 of the dome-like switch contact 27. When the depressing pressure is applied to the input button 21, since the center line 31 of the input button 21 is deviated from the center line 32 of the dome-like switch contact 27, the input button 21 is inclined in a direction opposite to the deviating direction of the dome-like switch contact 27 from the input button 21 as shown in FIG. 11, with the result that the dome-like switch contact 27 is pushed down from an slant upper position. Furthermore, a center of a depressing force transmitting region is shifted to the neighborhood of a point of action 35 shown in FIG. 11, so that the dome-like switch contact 27 is not sufficiently collapsed.
In the prior art structure, therefore, it is an indispensable condition to align respective center lines of three parts, namely, the input button 21, the projection 22, and the dome-like switch contact 27. The reason for this is that, since the prior art has the structure of electrically interconnecting the electrodes A 29 and B 30 by the dome-like switch contact 27, a depression of a center of the dome-like switch contact 27 in a vertical direction is a necessary condition for deforming the dome-like switch contact 27 in an ideal fashion.
Because of a structure restriction of the dome-like switch contact, if a deviation in a depressing force applying position and angle exceeds a given range of tolerance, the pushing pressure applied to the dome-like switch contact 27 from the input button cannot sufficiently overcome rigidity of the dome-like switch contact 27, so that the dome-like switch contact 27 is not sufficiently deformed, with the result that the contact becomes insufficient or poor.
The dome-like switch contact is effective in thinning the input button structure. However, the location of the input buttons greatly depends upon the position of the dome-like switch contacts located on the substrate. Therefore, the layout of the dome-like switch contacts restricts changes in appearance and precludes easy model changes. Thus, use of common parts and the degree of freedom in appearance are hindered.